The present invention relates to new and useful improvements in seals for fluid needle valves.
Equipment design often calls for the use of seals about moving parts to prevent the flow of fluid from one area of the apparatus into another. One such use is in the spray paint industry. Spray guns use seals about their fluid needles to prevent the migration of paints and other abrasives into the internal working of the gun where considerable wear and damage can occur. These seals must be able to withstand wide ranges of pressure and have long service lives.
One type of commercial seal presently available is the chevron "V" packing seal. This packing material takes the shape of a chevron or "V" and can be made of any of a number of materials. Normally, this type of seal requires manual adjustment when first inserted into the void between the needle or spindle and the housing to pre-seat the packing. The pre-seating involves the tightening of a gland-nut to compress and seat the packing to initiate the seal. The gland-nut is then loosened until the needle moves freely. Two obvious disadvantages with this type of seal are that pre-seating is required and that the adjustment of the packing is dependent upon the operator's sense of touch or feel.
Another problem is maintaining a tight seal once the packing is installed. Existing "V" type packing sets often have some pockets or cavities spaced along the length of their inside diameters. As the packing material wears, the remaining packing material has to be further compressed to maintain a tight seal. As this wearing occurs, paint is allowed to migrate into these pockets and becomes trapped. As the fluid needle continues to reciprocate, abrasive particles of paint begin to wear the nnedle surface. This continues until a massive leak occurs, at which time both the packing and the needle may be damaged.
Other types of prior art seals include those shown in Banks, U.S. Pat. No. 3,096,096 and Malone U.S. Pat. No. 3,085,628.
The present invention comprises a uniquely designed seal assembly including an insert or stop member and packing assembly which encircle the fluid needle valve. For purposes of illustration the drawings display the seal within a spray gun and as part of a fluid shut-off valve. These specific embodiments are for illustration purposes only and do not limit the use of such seal assemblies in other areas.
The material used to construct the insert or stop member of the present invention is largely dependent upon the design criteria of the equipment within which the seal assembly will be used. Within the present embodiments, the insert is constructed of brass. The exterior of the insert can be any shape necessary to accommodate the dimensions of the equipment within which the fluid needle seal assembly is placed. The interior of the insert includes a tapered annular surface which is designed to accept the packing member.
The angle of the tapered annular inner surface is dependent upon the type of packing material being used, the cold flow characteristics of the packing material and the amount of force exerted against the rear of the packing to urge the packing into contact with the fluid needle. Presently, there are a number of packing materials available, including polyethylene, polypropylene, vinyl, felt, and fluorocarbons such as Teflon and Teflon impregnated felt material. Within the present embodiments, a flurocarbon is used. It has been found that with a Teflon material the preferred angle of taper within the insert or stop member is approximately 15 degrees from the longitudinal axis of the fluid needle.
The packing member itself has a constant inside diameter which is slightly larger than the outside diameter of the needle, thus allowing needle movement while maintaining a tight seal. The outside of the packing member is preferrably stepped or two-tiered, with the outer diameter of the front of the packing member being smaller than the rear.
The front of the packing member, which fits into the tapered annular surface of the stop member insert, is preferrably a sharply cut-off tubular end. Positioned behind the packing member is a back-up washer and a spring. The spring acts against the back-up washer which in turn axially biases the packing member against the tapered annular surface of the insert to force the packing into contact with the fluid needle, thus creating a tight seal.
The advantages of the present seal assembly are many. The first advantage is the ease of assembly and replacement. There is no pre-seating of the packing required and adjustment is not subject to operator error. The operator simply installs a seal assembly and the spring continuously applies the force needed to create a proper seal. As a result, no adjustment is needed throughout the life of the packing. Once the packing reaches the end of its useful life, the operator simply removes the old packing and installs a new one.
Another advantage lies in the unique design and the resultant interface of the packing material and the tapered annular surface of the stop member. Most commercial seals require periodic adjustment because they rely on a replacement of packing material that has been worn away by compressing new material into the void. Existing "V" type packings rely on a reduction of the packing inner diameter. This reduction takes place only if movement of each packing section exists. This movement causes a wedge to spread the packing lip towards the center line, thus decreasing the packing inner diameter. Proper design practice, as recommended by packing manufacturers, dictates the use of a tight fit between the packing outer diameter and the housing. This tightness inhibits packing section movement thus resulting in early packing leakage. When this occurs, additional adjustments to the gland-nut are required.
The seal assembly, according to the present invention, does not rely on outer diameter packing movement for a tight seal. Instead, all the forces applied to the packing material are directed towards the reciprocating fluid needle. This is accomplished by minimizing the contact area between the packing and the insert through the use of the tapered annular surface of the stop member and the sharply cut-off tubular front end of the packing. The resultant seal between the Teflon packing and the metal needle occurs over a very small longitudinal portion of the needle. This results in the application of the major sealing forces at the very front edge of the seal only, which in turn reduces the amount of friction along the remainder of the packing.
In contrast, a standard chevron "V" packing distributes its force equally along each side of the packing. This results in higher friction along the entire length of the packing and a lower sealing force per unit of area against the needle for an equivalent spring force.
Finally, advantages lie in the increased useful life of each packing. Due to the seal assembly's unique design, its useful life has been found to exceed other commercially available seals by a factor of up to five to one .